1. Field of the Invention
The present invention relates generally to an apparatus and method for detecting feedback information in a broadband wireless communication system, and in particular, to an apparatus and method for detecting uplink fast feedback information using data symbols and pilot symbols included in the uplink fast feedback information in a broadband wireless communication system.
2. Description of the Related Art
In a high-speed mobile communication system, a Base Station (BS) schedules packet data transmission and determines transmission parameters using uplink fast feedback information representing downlink channel quality, to thereby provide high-speed packet data service to Mobile Stations (MSs). Upon receipt of the uplink fast feedback signals from the MSs, the BS checks the downlink channel statuses to the MSs based on the feedback information. The BS then selects MSs having the best downlink channel quality according to the channel status information in slot and sends packet data to them. The BS also determines transmission parameters (e.g. data rate, code rate, and modulation order) according to the downlink channel quality of the selected MSs. The uplink fast feedback information may include a Signal-to-Noise Ratio (SNR), a Carrier-to-Interference Ratio (C/I), the differential SNR of each band, a fast Multiple Input Multiple Output (MIMO) feedback, or a mode selection feedback. Additional physical channels are configured for delivering the uplink fast feedback information in an Orthogonal Frequency Division Multiple Access (OFDMA) communication system.
The BS necessarily uses C/Is measured at the MSs in determining the downlink channel quality. The MSs measure C/Is and feed back the C/I measurements to the BS on physical channels, e.g. Channel Quality Indicator CHannels (CQICHs). The BS schedules downlink data for the MSs and determines transmission parameters based on the C/I measurements.
The C/I information, by which downlink data rates and cell throughput are determined, has to be sent with high reliability despite its small size, because it is very critical to the operation of the communication system. Yet, it is typical not to allocate much time-frequency resources to the physical channels carrying the fast feedback information in order to reduce overhead. Accordingly, there exists a need for an efficient detection method to enable reliable transmission.
The configurations of channels for delivering the fast feedback information are illustrated in FIGS. 1 and 2. The channels are an Optional Partial Usage of SubChannel (OPUSC) in FIG. 1 and a Partial Usage of SubChannel (PUSC) in FIG. 2, respectively.
Referring to FIG. 1, six tiles 100 are allocated, each tile having a set of 3×3 subcarriers 110 on a frequency-time domain. Eight surrounding subcarriers deliver modulation symbols (MOD SYM in FIG. 1) and the remaining center subcarrier delivers a pilot symbol in the 3×3 subcarrier set 110.
Referring to FIG. 2, six tiles 200 are allocated, each tile having a set of 4×3 subcarriers 210 on the frequency-time domain. Four corner subcarriers deliver pilot symbols and the remaining eight subcarriers deliver modulation symbols (MOD SYM in FIG. 1) in the 4×3 subcarrier set 210.
In the case where an MS sends uplink fast feedback information on either of the channels illustrated in FIGS. 1 and 2, upon generation of uplink fast feedback information data, it modulates a codeword corresponding to the feedback information data in a non-coherent manner and sends the modulation symbols. For example, for the input of 4-bit information data, the MS creates a codeword (0˜15 for each tile A0, Al, A2, A3, A4 and A5) as illustrated in FIG. 3. The MS maps orthogonal vectors corresponding to the codeword to predetermined tiles by non-coherent modulation. Specifically, the MS maps the orthogonal vector corresponding to each codeword of A0 to a first tile, the orthogonal vector corresponding to each codeword of A1 to a second tile, and the orthogonal vector corresponding to each codeword of A2 to a third tile. In the same manner, the MS maps the orthogonal vector corresponding to each codeword of A5 to the last tile through non-coherent modulation. Orthogonal vectors corresponding to codewords are illustrated in FIG. 5.
As described above, FIGS. 3 and 4 illustrate codewords that can be generated from an M-ary channel encoder. 24(=16) codewords that are generated for the input of four information data bits are shown in FIG. 3, while 26(=64) codewords that can be generated for the input of six information data bits are shown in FIG. 4. Orthogonal vectors used for orthogonal modulation of the codewords are listed in FIG. 5.
      P    ⁢                  ⁢    0    ⁢          (              exp        ⁡                  (                      j            ⁢                          π              4                                )                    )        ,      P    ⁢                  ⁢    1    ⁢          (              exp        ⁡                  (                      j            ⁢                                          3                ⁢                π                            4                                )                    )        ,      P    ⁢                  ⁢    2    ⁢          (              exp        ⁡                  (                                    -              j                        ⁢                                          3                ⁢                π                            4                                )                    )        ,      and    ⁢                  ⁢    P    ⁢                  ⁢    3    ⁢          (              exp        ⁡                  (                                    -              j                        ⁢                          π              4                                )                    )      are Quadrature Phase Shift Keying (QPSK) symbols. Pilot symbols, which are known to both the BS and the MS, are generally multiplied by a scrambling code and modulated in Binary Phase Shift Keying (BPSK).
Now, reception of the uplink fast feedback information sent in the above-described manner is described. The BS, upon receipt of the uplink fast feedback information, decides soft-decision values of the symbols of the feedback information by non-coherent demodulation. Then the BS determines a codeword corresponding to the soft-decision values and outputs the information data bits corresponding to the codeword.
In the case of 4-bit information data, for example, upon receipt of feedback information, the BS correlates modulation symbols on eight subcarriers in each of the six tiles where the feedback information is received as illustrated in FIG. 1 with orthogonal vectors (symbols) corresponding to each codeword for the tiles, and calculates the square of the absolute value of the correlation as expressed by the following Equation (1):
                                                                  ∑                              k                =                1                                            #                ⁢                                                                  ⁢                of                ⁢                                                                  ⁢                Fast                ⁢                                                                  ⁢                Feedback                ⁢                                                                  ⁢                tones                ⁢                                                                  ⁢                per                ⁢                                                                  ⁢                tile                ⁢                                                                  ⁢                                  (                                      =                    8                                    )                                                      ⁢                                          X                k                *                            ⁢                              Y                k                                                              2                            (        1        )            where Xk denotes a orthogonal vector (symbol) corresponding to a codeword on a kth subcarrier, Yk denotes a symbol received on the kth subcarrier in the tile, k denotes a subcarrier index and superscript * denotes complex conjugate operation.
For each of all 16 codewords that are generated from 4-bit information data, the BS sums the squares of the absolute values of the correlations of all 16 codewords for all 6 tiles, then selects a codeword corresponding to the maximum of the sums. The BS determines information data corresponding to a codeword having the maximum sum to be feedback information transmitted by the MS.
To render the detection reliable, the average of the sums with respect to the 16 codewords for all 6 tiles is calculated. If the difference between the maximum and the average exceeds a predetermined threshold, the BS determines that the detected information data bits are reliable.
The process of detecting uplink fast feedback information is expressed as Equation (2):
                                                        max              n                        ⁢                          {                                                                    ∑                                          m                      =                      1                                                              #                      ⁢                                                                                          ⁢                      of                      ⁢                                                                                          ⁢                      tiles                      ⁢                                                                                          ⁢                                              (                                                  =                          6                                                )                                                                              ⁢                                                                          ⁢                                                                                                                                    ∑                                                      k                            =                            1                                                                                #                            ⁢                                                                                                                  ⁢                            of                            ⁢                                                                                                                  ⁢                            Fast                            ⁢                                                                                                                  ⁢                            Feedback                            ⁢                                                                                                                  ⁢                            tones                            ⁢                                                                                                                  ⁢                            per                            ⁢                                                                                                                  ⁢                            tile                            ⁢                                                                                                                  ⁢                                                          (                                                              =                                8                                                            )                                                                                                      ⁢                                                                                                  ⁢                                                                              X                                                          n                              ,                              m                              ,                              k                                                        *                                                    ⁢                                                      Y                                                          m                              ,                              k                                                                                                                                                                  2                                                  ,                                  n                  =                  0                                ,                1                ,                …                ⁢                                                                  ,                15                            }                                -                                          ⁢                                    avrg              n                        ⁢                          {                                                                    ∑                                          m                      =                      1                                                              #                      ⁢                                                                                          ⁢                      of                      ⁢                                                                                          ⁢                      tiles                      ⁢                                                                                          ⁢                                              (                                                  =                          6                                                )                                                                              ⁢                                                                          ⁢                                                                                                                                    ∑                                                      k                            =                            1                                                                                #                            ⁢                                                                                                                  ⁢                            of                            ⁢                                                                                                                  ⁢                            Fast                            ⁢                                                                                                                  ⁢                            Feedback                            ⁢                                                                                                                  ⁢                            tones                            ⁢                                                                                                                  ⁢                            per                            ⁢                                                                                                                  ⁢                            tile                            ⁢                                                                                                                  ⁢                                                          (                                                              =                                8                                                            )                                                                                                      ⁢                                                                                                  ⁢                                                                              X                                                          n                              ,                              m                              ,                              k                                                        *                                                    ⁢                                                      Y                                                          m                              ,                              k                                                                                                                                                                  2                                                  ,                                  n                  =                  0                                ,                1                ,                …                ⁢                                                                  ,                15                            }                                      >        threshold                            (        2        )            where Ym,k denotes a symbol received on a kth subcarrier in an mth tile, Xn,m,k denotes a orthogonal vector (symbol) corresponding to an nth codeword for the kth subcarrier in the mth tile, n denotes a codeword index (e.g. 24(n=0, . . . , 15)), m denotes a tile index (m=0, 1, 2, 3, 4, 5), and k denotes a subcarrier index (k=0, 1, . . . , 7) and superscript * denotes complex conjugate operation.
Here, information data corresponding to a codeword having the maximum of the sums of the squares of the absolute values of the correlations is deemed to be transmitted as feedback information.
As described above, an uplink subchannel carries uplink fast feedback information. To detect information bits, received symbols are decoded using only orthogonal vectors corresponding to a codeword, without utilizing full knowledge of the subchannel carrying the uplink fast feedback information. In this context, the conventional detection method may be improved.